The content of dissolved salts in brackish water is higher than the fresh water and ranges between 0.5 to 35%. These waters of intermediate salinity range are distinct from fresh or marine waters. In estuary which represents the tail end of a river, mixing of fresh water with sea water results in brackish water. The salinity in an estuary increases from upper middle to lower reaches and at the mouth of the estuary the salinity is nearly equal to that of the sea.
The sea water is highly salty. The average salinity of sea water remains almost constant at 35 parts of salt per 1000 parts of water by weight and written as 35%. Some salt lakes may also have salinity between 25% to 35%. The biotic activity in such habitats is greatly restricted. The absence of many fresh water animals and plants from the marine environment is largely due to their inability to tolerate the high salinity of the sea water. Insects are mostly absent from marine environment except Halobates and shore collembolans like Isotoma and Sminthurus.